A. F. Carlsson

The adsorption and reaction of low molecular weight alkanes on metallic single crystal surfaces

The adsorption and reaction of alkanes on/with metal single crystal surfaces are reviewed in this report. A comprehensive treatment of the structures and binding of molecular alkanes on metals is first presented. A detailed discussion of the current state of understanding of the dynamics of alkane trapping at surfaces then follows, including the most recent treatments of the roles of both intrinsic and extrinsic precursor states and the influence of preadsorbed species on trapping. The progress in the use of molecular dynamics simulations to predict trapping probabilities is thoroughly discussed. The current states of both the dynamics and the kinetics of dissociative adsorption on a variety of metal surfaces are elaborated.

Molecular adsorption and growth of n-butane adlayers on Pt(1 1 1)

The molecular adsorption of n-butane and the growth of n-butane adlayers on Pt(1 1 1) was investigated using molecular beam techniques, temperature-programmed desorption (TPD) and low-energy electron diffraction (LEED). It is found that as the surface coverage of n-butane increases, structural changes occur in the adlayer at surface temperatures near 98 K that are accompanied by changes in the binding energy and mobility of the adsorbed species. The film growth process can be divided into four distinct coverage regimes. At low coverages (0 < 0.14 ML, where 1 ML is defined as one butane molecule per Pt atom) a disordered monolayer forms in which the butane molecules prefer to lie parallel to the surface in order to minimize their binding energy. At coverages from 0.14 to 0.20 ML, ordered regions develop within the monolayer in which the butane molecules also lie parallel to the surface. The binding energy in the ordered phase is lower than that in the disordered phase due to repulsive intermolecular interactions. A more densely-packed ordered phase begins to form at 98 K after the low-coverage ordered phase saturates at 0.20 ML. The experimental results suggest that the n-butane molecules tilt away from the surface in the high-coverage ordered phase. Finally, a disordered second layer phase forms after the high coverage ordered phase saturates at 0.35 ML. The molecules in the second layer are very mobile at 98 K and rapidly diffuse to the edges of the beam spot. Interchange of molecules between the second layer and ordered monolayer is found to govern the net rate of second layer diffusion at surface temperatures less than 133 K. The adsorption probability of n-butane on Pt(1 1 1) continuously increases with increasing coverage, with no significant dependencies on the structure of the n-butane adlayer. This finding indicates that the long-range arrangements and molecular orientations of a mobile alkane adlayer have a negligible influence on the intrinsic adsorption dynamics, suggesting that the energy transfer processes that facilitate adsorption are highly localized.

The dynamics of ethylene adsorption on Pt(111) into di-σ and π-bonded states

The dynamics of ethylene adsorption on Pt(111) into both the di-σ- and π-bonded states were investigated at 95 and 40 K, respectively, using supersonic molecular beam techniques. The angular dependence of ethylene adsorption into both states is similar to the angular dependence for ethane adsorption, which has a much weaker bond to the surface in its final state. In contrast to ethane, high adsorption probabilities for ethylene prevail to high incident kinetic energies, suggesting that the strong interaction of ethylene with the surface influences adsorption. The initial adsorption probability of ethylene is approximately independent of surface temperature between 40 and 450 K, suggesting that there is no reversible, thermalized intrinsic precursor to adsorption. At 40 K, the adsorption probability increases with coverage (in the π-bonded state). However, at 95 K, the adsorption probability of ethylene remains constant with increasing self-coverage (in the di-σ- bonded state) for trajectories incident wit...

Intrinsic and extrinsic precursors to adsorption: Coverage and temperature dependence of Kr adsorption on Pt(111)

The kinetics of krypton adsorption on Pt(111) were investigated using supersonic molecular beam techniques. Krypton adsorbs at defects via an intrinsic precursor below a surface temperature of 85 K. The difference in activation energies for desorption and migration of a Kr atom on the terrace seeking a defect site is 10.7 kJ/mol, indicating that at 80 K, a Kr atom makes about 107 site hops before desorbing or finding a binding site. Below 60 K stable adsorption occurs on terraces, where the initial adsorption probability is independent of surface temperature. The activation energy for zero-order desorption from Pt(111) terraces is 12.9 kJ/mol; the activation energy for Kr migration on the terraces is then calculated to be ⩽2.2 kJ/mol. Krypton adsorption proceeds at nonzero coverages via an extrinsic precursor. The adsorption probability of Kr increases with self-coverage, and is described by the modified Kisliuk model [H. C. Kang, C. B. Mullins, and W. H. Weinberg, J. Chem. Phys. 92, 1397 (1990); C. R. Ar...

Trapping of Ar on well ordered Ar, Kr, and Xe overlayers on Pt(111) at 30 K

The dynamics of Ar trapping on Ar, Kr, and Xe covered Pt(1 1 1) were investigated using supersonic molecular beam techniques at a surface temperature of 30 K. The initial trapping probability of Ar on the clean surface decreases from 0.8 to 0 as the normal incident energy is increased from 1.75 to 28 kJ mol−1, and scales with normal incident energy (ETcos2θ), indicating a smooth gas–surface potential [A.F. Carisson R.J. Madix, submitted for publication]. In contrast, the trapping probability on the Ar, Kr and Xe saturated surfaces decreases from 0.9 to only 0.3 in the same energy range and is nearly independent of the angle of incidence. The relative trapping probability on the Ar and Xe covered surfaces is in general accord with the relative energy exchange expected from the simple Baule model, but the trapping probability on the Kr covered surface appears further enhanced by potential corrugation. Experimental results for Ar trapping on Pt(1 1 1)–Ar are in qualitative agreement with a recoil effect predicted by Head-Gordon and Tully for Ar trapping on Ar covered Ru(0 0 1) [Surf. Sci. 268 (1992) 113]. At high incident energies on the gas covered Pt(1 1 1) surface, the adsorption probability is nearly independent of angle of incidence between 0° and 30°, but it then increases rapidly between 30° and 60°. Similar effects are observed for Ar trapping on Pt(1 1 1) saturated by Kr and Xe. The dependence of the trapping probability on coverage is quantitatively described by the modified Kisliuk model [J. Chem. Phys. 92 (1990) 1397; J. Phys. Chem. 95 (1991) 2461], which allows for an extrinsic precursor to adsorption as well as a direct channel. The probability of Ar trapping increases with Ar uptake to different degrees on the Ar, Kr and Xe covered surfaces.

The trapping of methane on ordered structures of CO on Pt(111)

The dynamics of methane trapping on CO-covered Pt(111) in low coverage, c(√3×5)rect, and c(4×2) structures was investigated using supersonic molecular beam techniques at a surface temperature of 50 K; at this temperature methane was stably adsorbed on the clean (A. F. Carlsson and R. J. Madix, to be published) surface, but not in multilayers (A. F. Carlsson and R. J. Madix, to be published), and thus trapped amidst adsorbed CO molecules. Molecular trapping was enhanced to greater degrees with increasing CO coverage, and the methane uptake decreased with increasing CO coverage, as would be expected. The trapping probability further increased as methane covered the Pt(111)–CO surface; the modified Kisliuk model [J. Chem. Phys. 92, 1397 (1990); J. Phys. Chem. 95, 2461 (1991)] describes the coverage-dependent trapping probability. Methane adsorption may occur directly on the surface, or via two entrance channels into an extrinsic precursor, where the trapping probability is higher. The angular dependence of m...

Molecular ethane adsorption dynamics on oxygen-covered Pt(111)

The dynamics of ethane trapping on Pt(111)-p(2×2)-O were investigated by supersonic molecular beam techniques at a surface temperature of 100 K. The initial trapping probability was measured in the range of incident energy from 10 to 45 kJ/mol and incident angles from 0° to 60°. A broad angular distribution of scattered ethane and total energy scaling (ET cos 0.2θ) for ethane trapping indicated a corrugated gas–surface potential. Stochastic trajectory simulations employing a potential developed from the trapping of ethane on Pt(111) gives quantitative agreement of the measured initial trapping probabilities over entire ranges of incident energies and angles. Calculations of energy transfer for ethane after the first bounce on Pt(111) and Pt(111)-p(2×2)-O clearly indicate that interconversion of parallel and perpendicular momentum and energy transfer to lattice vibrations account primarily for the differences in trapping probabilities between ethane on the two surfaces. At glancing incidence trapping is not significantly reduced on the oxygen-covered Pt(111) because the parallel momentum appears to be transferred partially to phonons.